Enhanced silicon all-optical modulator

ABSTRACT

A single-photon absorption all-optical signal-processing device, systems employing the same, and methods of making and using the same. Illustrative examples are provided based on silicon semiconductor technology that employs rectangular waveguides fabricated on SOI wafers. In some embodiments, it is observed that the waveguides have surface state density, σ, of not less than 1.5×10 18  cm −1 s −1 mW −1  to provide a single-photon absorption operation mode. In some embodiments, some portion of the ridge waveguide structure has a surface to volume ratio of at least 18 μm −1 , computed using a unit length of 1 μm of the waveguide, with the width and depth dimensions of the waveguide being measured in units of microns.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Patent Application No. PCT/US09/33516, filed Feb. 9, 2009, which claims priority to and the benefit of U.S. provisional patent application Ser. No. 61/027,022, filed Feb. 7, 2008, and which each application is incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY FUNDED RESEARCH OR DEVELOPMENT

The U.S. Government has certain rights in this invention pursuant to Grant No, HR0011-04-1-0054 awarded by DARPA and Grant No. FA9550-04-1-0434 awarded by AFOSR.

FIELD OF THE INVENTION

The invention relates to all-optical modulators in general and particularly to a silicon all-optical modulator that employs single photon absorption.

BACKGROUND OF THE INVENTION

All-optical, low-power modulation is a major goal in photonics. Because of their high mode-field concentration and ease of manufacturing, nanoscale silicon waveguides offer an intriguing platform for photonics. To the best knowledge and belief of the inventor, all-optical modulators built with silicon photonic circuits have relied on either two-photon absorption (TPA) or the Kerr effect. Both effects are weak in silicon, and require extremely high (˜5 W) peak optical power levels to achieve modulation.

Almeida et al (“All-optical control of light on a silicon chip,” Nature, vol. 431, pp. 1081-1084, 2004.) and Foerst et al (“High-speed all-optical switching in ion-implanted silicon-on-insulator microring resonators,” Optics Letters, vol. 32, pp. 2046-2048, 2007) have recently demonstrated an all-optical modulator with single-photon absorption-based carrier injection using visible light. This approach has severe limitations. Because of the disparate wavelengths for gate and signal, these devices cannot be cascaded into circuits that require feedback.

There is a need for an all-optical modulator that uses the same wavelength of light to control the device as is used to carry signals representing information through the device.

SUMMARY OF THE INVENTION

In one aspect, the invention relates to an optical modulator configured to operate in a single-photon absorption mode. The optical modulator comprises a substrate configured to absorb photons and to create charge carriers, the substrate configured to accept application of a voltage in a desired direction, the voltage when applied configured to move the charge carriers along the desired direction; a waveguide structure having a gate waveguide and at least one signal waveguide having an input port and an output port, the waveguide structure disposed adjacent the substrate in an orientation selected with respect to the direction of voltage, at least one of the substrate and the waveguide structure configured to operate in a single-photon absorption mode; the gate waveguide of the waveguide structure configured to receive a gate signal; the input port of the at least one signal waveguide configured to receive an input optical signal to be controlled in response to the gate signal; and the output port of the at least one signal waveguide configured to provide as output an output signal representative of the controlled input signal.

In one embodiment, the optical modulator further comprises a ring resonator. In one embodiment, the at least one signal waveguide is configured as one leg of a Mach-Zehnder interferometer. In one embodiment, the waveguide structure comprises a semiconductor material. In one embodiment, the semiconductor material is silicon.

In one embodiment, the waveguide has surface state density, σ, of not less than 1.5×10¹⁸ cm⁻¹s⁻¹mW⁻¹. In one embodiment, the optical modulator further comprises a ring resonator. In one embodiment, the at least one signal waveguide is configured as one leg of a Mach-Zehnder interferometer. In one embodiment, the waveguide structure comprises a semiconductor material. In one embodiment, the semiconductor material is silicon.

In one embodiment, some portion of the waveguide structure having a surface to volume ratio of at least 18 μm⁻¹, computed using a unit length of 1 μm of the waveguide, with the width and depth dimensions of the waveguide being measured in units of microns. In one embodiment, the optical modulator further comprises a ring resonator. In one embodiment, the at least one signal waveguide is configured as one leg of a Mach-Zehnder interferometer. In one embodiment, the waveguide structure comprises a semiconductor material. In one embodiment, the semiconductor material is silicon.

In one embodiment, a material providing single-photon absorption is an alloy of one or more of silicon, germanium, and tin. In one embodiment, the material providing single-photon absorption comprises a dopant. In one embodiment, the optical modulator comprises a junction selected from a P-N junction and a P-I-N junction.

In another aspect, the invention features a method of operating an optical modulator in a single-photon absorption mode. The optical modulator comprises a substrate configured to absorb photons and to create charge carriers, the substrate configured to accept application of a voltage in a desired direction, the voltage when applied configured to move the charge carriers along the desired direction; a waveguide structure having a gate waveguide and at least one signal waveguide having an input port and an output port, the waveguide structure disposed adjacent the substrate in an orientation selected with respect to the direction of voltage, at least one of the substrate and the waveguide structure configured to operate in a single-photon absorption mode; the gate waveguide of the waveguide structure configured to receive a gate signal; the input port of the at least one signal waveguide configured to receive an input optical signal to be controlled in response to the gate signal; and the output port of the at least one signal waveguide configured to provide as output an output signal representative of the controlled input signal. The method comprises the steps of: applying to the input port of the at least one signal waveguide an input optical signal; applying to the gate waveguide a gate signal representing a desired modulation to be applied to the input optical signal; and detecting at the output port of the at least one signal waveguide an output signal representing the input signal modulated using the gate signal. In this manner, the input signal is modulated by the optical modulator.

In one embodiment, the optical modulator further comprises a ring resonator. In one embodiment, the at least one signal waveguide is one leg of a Mach-Zehnder interferometer. In one embodiment, the waveguide structure comprises a semiconductor material. In one embodiment, the semiconductor material is silicon.

In one embodiment, the waveguide has surface state density, σ, of not less than 1.5×10¹⁸ cm⁻¹s⁻¹mW⁻¹. In one embodiment, the optical modulator further comprises a ring resonator. In one embodiment, the at least one signal waveguide is configured as one leg of a Mach-Zehnder interferometer. In one embodiment, the waveguide structure comprises a semiconductor material. In one embodiment, the semiconductor material is silicon.

In one embodiment, some portion of the waveguide structure has a surface to volume ratio of at least 18 μm⁻¹, computed using a unit length of 1 μm of the waveguide, with the width and depth dimensions of the waveguide being measured in units of microns. In one embodiment, the optical modulator further comprises a ring resonator. In one embodiment, the at least one signal waveguide is configured as one leg of a Mach-Zehnder interferometer. In one embodiment, the waveguide structure comprises a semiconductor material. In one embodiment, the semiconductor material is silicon.

In one embodiment, a material providing single-photon absorption is an alloy of one or more of silicon, germanium, and tin, in one embodiment, the material providing single-photon absorption comprises a dopant. In one embodiment, the optical modulator comprises a junction selected from a P-N junction and a P-I-N junction.

The foregoing and other objects, aspects, features, and advantages of the invention will become more apparent from the following description and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention can be better understood with reference to the drawings described below, and the claims. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the drawings, like numerals are used to indicate like parts throughout the various views.

FIG. 1( a) through FIG. 1( c) illustrate an exemplary embodiment a waveguide that is useful to provide single photon absorption modulator device layout, with some of the properties of the modulator, according to principles of the invention;

FIG. 1( d) is a SEM micrograph of a detector device of type B;

FIG. 1( e) is another SEM micrograph of a device of type B;

FIG. 2 is a diagram showing an illustrative experimental setup used to measure the response of the single photon absorption modulator, and showing eye-patterns observed, according to principles of the invention;

FIG. 3 is a diagram showing the results of lock-in amplifier assisted measurements, according to principles of the invention;

FIG. 4 is a graphical representation of the frequency dependant device response at speeds up to approximately 50 MHz, according to principles of the invention;

FIG. 5 is a graphical diagram illustrating the high speed frequency response of a single photon absorption modulator device, according to principles of the invention;

FIG. 6 is a diagram illustrating in graphical form the results of pulsed measurements observed using a single photon absorption modulator, according to principles of the invention;

FIG. 7 is a diagram that illustrates projected performance of single photon absorption devices for varying surface-state absorption densities, according to principles of the invention;

FIG. 8( a) is a diagram of a detector of type A;

FIG. 8( b) is a diagram of a detector of type B;

FIG. 8( c) is a diagram of an equivalent circuit;

FIG. 8( d) is a diagram of an experimental setup;

FIG. 9( a) is a graph showing DC I-V curves for device A1;

FIG. 9( b) is a graph showing DC I-V curves for device B1;

FIG. 9( c) is a diagram showing the peak to peak output photocurrent of device B2 as a function of frequency;

FIG. 10( a) is a graph showing the photocurrent as a function of propagating laser power for several bias voltages;

FIG. 10( b) is a graph showing the photocurrent in peak to peak nA of the device for a 11 V bias voltages and several peak to peak laser powers as a function of frequency up to 1 MHz;

FIG. 11( a) is a diagram in plan of a prior art optical modulator fabricated using ion implantation to produce damage in a silicon waveguide;

FIG. 11( b) is a diagram in elevation of the prior art optical modulator illustrated in FIG. 11( a);

FIG. 12 is a diagram in perspective illustrating the structure and operation of a portion of an all-optical transistor;

FIG. 13( a) is a diagram in side view of a portion of an illustrative all-optical transistor;

FIG. 13( b) is a diagram in plan view of a portion of an illustrative all-optical transistor;

FIG. 13( c) is a schematic diagram illustrating the logical layout of an illustrative all-optical transistor;

FIG. 14 is a schematic plan diagram showing the layout of an all-optical transistor that comprises a Mach-Zehnder interferometer geometry, in which one arm of the Mach-Zehnder is the signal waveguide of FIG. 13( a) and FIG. 13( b); and

FIG. 15 is a schematic plan diagram showing the layout of an all-optical transistor that comprises a ring resonator geometry.

DETAILED DESCRIPTION OF THE INVENTION

Because silicon has a bandgap of 1.12 eV, it is an ideal material platform for near-infrared integrated optical circuits. Electrically driven modulation and an optically pumped silicon laser have been previously demonstrated. For all-optical signal processing applications, low-power all-optical modulation is very useful and would represent an appreciable advance. Applications that are contemplated include optical buffering, all-optical wavelength conversion, and all-optical computation.

Silicon is an extremely attractive material platform for integrated optics at telecommunications wavelengths, particularly for integration with CMOS circuits. Developing detectors and electrically pumped lasers at telecom wavelengths are the two main technological hurdles before silicon can become a comprehensive platform for integrated optics. We describe a photocurrent in unimplanted SOI ridge waveguides. It is believed that the photocurrent is a consequence of surface state absorption. By electrically contacting the waveguides, a photodetector with a responsivity of 36 mA/W and quantum efficiency of 2.8% is demonstrated. The response is shown to have minimal falloff at speeds of up to 60 MHz.

Silicon is also a useful material because of its low cost as compared to many other semiconductors, and because it has a very well developed and well understood processing technology. Nevertheless, other semiconductor materials might in principle be used instead of silicon in a single-photon absorption device. For some optical wavelengths, other materials might be advantageous as compared to silicon.

All-Optical Mach-Zehnder Modulator

We describe an all-optical Mach-Zehnder modulator based on a single-photon absorption (SPA) process, fabricated entirely in silicon. Our single-photon absorption modulator is based on a process by which a single photon at 1.55 μm is absorbed and an apparently free-carrier mediated process causes an index shift in silicon, even though the photon energy does not exceed that of silicon's bandgap. We demonstrate all-optical modulation with a gate response of 1 degree/mW at 0.5 GBit/s. This is over an order of magnitude more responsive than typical previously demonstrated devices. Even without resonant enhancement, further engineering may enable all optical modulation with less than 10 mW of gate power required for complete extinction, and speeds of 5 GBit/s or higher.

We have demonstrated all-optical modulation in silicon with a novel single photon mechanism. Our device achieves modulation at power levels an order of magnitude lower than typical all-optical modulators in silicon. We believe that bandwidths in the tens of gigahertz and peak modulation powers on the scale of 10 mW are achievable. With such performance, it will be possible to obtain broadband all-optical signal gain, enabling chip-scale optical transistors to be integrated into all-optical integrated logic circuits.

Geis et al. (“CMOS-Compatible All-Si High-Speed Waveguide Photodiodes With High Responsivity in Near-Infrared Communication Band,” IEEE Photonics Technology Letters, vol. 19, pp. 152-154, 2007) demonstrated an efficient photodetector at 1.55 μm based on an single-photon absorption mechanism, achieving greater than 50% quantum efficiency at speeds up to 10 GHz. This photodetector utilized absorption centers created in a waveguide by ion damage (e.g., implanting silicon with silicon ions), which enabled absorption of photons at energies below the bandgap, which absorption appears to be correlated with the defects in the volume of the semiconductor. Single photon absorption was also observed in undamaged samples, and attributed to surface-state absorption. The photodetector is illustrated in FIG. 10( a) and FIG. 10( b). Similar linear photocurrent responses have been observed, both due to volume defects and due solely to the waveguide surface states, for example in J. D. B. Bradley, et al., “Silicon waveguide-integrated optical power monitor with enhanced sensitivity at 1550 nm,” Applied Physics Letters, vol. 86, art. no. 241103, 2005, in Y. Liu et al., “In-line channel power monitor based on helium ion implantation in silicon-on-insulator waveguides,” IEEE Photonics Technology Letters, vol. 18, pp. 1882-1884, 2006, and in T. Baehr-Jones, M. Hochberg, and A. Scherer, “Photodetection in silicon beyond the band edge with surface states,” Optics Express, vol. 16, pp. 1659-1668, 2008. It has been hypothesized that defects create mid-bandgap states, enabling an electron to reach the conduction band from the valence hand, but the precise mechanism is not yet fully understood. These investigations did not deal with modulation effects, but rather simply with photodetection.

It is well known that surface states cause optical loss in silicon waveguides. Most low-loss geometries involve large silicon waveguides, on the scale of 0.450 μm×0.250 μm and 2 μm×0.9 μm, which minimize the interaction of the optical mode with surface states. In our singe-photon absorption modulator, we use a smaller 0.5×0.1 μm ridge waveguide resulting in a very large electric field at the etched surfaces. The fabrication of ridge waveguides in silicon, such as the 0.5×0.1 μm ridge waveguide, has previously been described in several of our previous patent documents, including U.S. Pat. Nos. 7,200,308 and 7,424,192, and in published application US2007/0035800 A1, all of which are incorporated by reference herein in their entirety. By electrically contacting the silicon waveguides, we have demonstrated that a linear photocurrent can be observed, with a quantum efficiency of 2.8%. We have identified that the region responsible for the photocurrent was the waveguide surface, though the precise mechanism was not determined.

Mach-Zehnder Device Layout and Test Layout

We use the surface-absorption process to build an all-optical modulator. We introduce a gate optical mode into one arm of a Mach-Zehnder interferometer. The single-photon absorption process occurs, and an unbalanced refractive index shift occurs in one of the arms, causing constructive or destructive interference in a signal beam that is provided to the input port of the Mach-Zehnder interferometer, with the output observed at an output port of the Mach-Zehnder interferometer.

FIG. 1( a) is diagram showing a SEM micrograph of cleaved ridge waveguide with modal patterns illustrated to the left and with calculated electrical field contours shown to the right. FIG. 1( b) is a diagram showing an optical image of the single-photon absorption modulator, with the gate and signal optical modes illustrated. P_(gate) indicates the location that the gate optical mode is mixed into a Mach-Zehnder arm with a y-junction, and begins to cause a phase delay, labeled Δφ, in the signal mode. The input port to the Mach-Zehnder is indicated by the port an the lower right of the figure having an arrow pointing thereto, indicating that the input signal is applied to that port. The output port of the Mach-Zehnder is the port in the bottom middle of the figure having two arrows pointing away therefrom, indicating that the output signal is provided at that port. The output port of the Mach-Zehnder is so labeled explicitly in FIG. 2( a). The gate power as reported in the paper is always the propagating power at this point in the waveguide. Thought not visible in this image, there is a corresponding y-junction on the opposing arm, ensuring that the Mach-Zehnder is balanced. FIG. 1( c) is a diagram showing the idealized transmission of the signal is shown, for a device with a gate response of 1 degree/mW, and a bias point of 90 degrees.

Referring to FIG. 1, showing the device geometry, the arm lengths of the devices that we tested ranged from 0.75 to 1.25 cm, and the arms were unequal in length within each device, with differences ranging from 150 μm to 600 μm. This length inequality allows us to control the intrinsic phase shift between, the arms by tuning the signal wavelength. Typical waveguide losses in these devices were 6 dB/cm. Input coupling is achieved from a polarization-maintaining fiber array via grating couplers.

The device performance is best characterized by the phase shill induced per unit gate power. A gate response of 1 degree/mW corresponds to around 180 mW for complete extinction of the signal mode, if the gate response remains linear at higher powers. The power level required to obtain extinction of the signal, which we call Pπ, is analogous to the Vπ associated with an electrooptic modulator.

Fabrication Information

Devices were fabricated in electronics-grade silicon-on-insulator (SOI) wafers supped by Soitec (Soitec USA Inc., 2 Centennial Drive, Peabody, Mass. 01960, http://www.soitec.com/en/index.php), doped at around 10¹⁵ dopant atoms (Boron)/cm³. No implant or irradiation was performed on the silicon material. The starting silicon material was thinned to about 110 nm by dry oxidation, separated into small chips, and patterned with electron-beam lithography using a 100-kV electron-beam writer using hydrogen silsesquioxane (HSQ) resist. The samples were etched with chlorine in an inductively coupled plasma etcher. No cladding layer was deposited. The SOI wafer serves as a substrate for the fabricated devices.

Testing with Pseudo-Random Bit Sequence

Initial testing was done by modulating the gate beam with a pseudo-random bit sequence. A modulated gate optical mode on the order of 25 mW propagating power in the waveguide with 50% extinction was used, and this was directed into the gate port of the modulator. To assist the measurements and enhance the effect by increasing the optical power levels, two Erbium Doped Fiber Amplifiers (EDFA) were used. The signal beam was set to a wavelength such that there was an intrinsic 90 degree modulator bias point. We obtained eye diagrams at 300 and 500 MBit/sec, as shown in FIG. 2.

FIG. 2( a) is a diagram showing an illustrative experimental setup used to measure the response of the single-photon absorption modulator, and used to obtain an eye-pattern. FIG. 2(b) is a diagram showing an eye-pattern obtained at 300 MBit/sec. FIG. 2( c) is a diagram showing an eye-pattern obtained at 500 MBit/see.

As shown in FIG. 2( a), the gate is supplied with a signal generated using a laser source feeding a signal to an electro-optical modulator, such as a lithium niobate (LiNbO₃) electro-optical modulator that is driven by an electrical pseudorandom binary sequence (PRBS) generator. The signal from the electro-optical modulator is amplified by an EDFA, and applied to the gate port. The input signal applied to the input port of the Mach-Zehnder is generated using a laser signal that is amplified by an EDFA. The output from the Mach-Zehnder is observed after being passed through a spectral filter, and detected with a photodiode that provides an electrical signal representative of the output of the Mach-Zehnder. The electrical signal is amplified with an RF amplifier, and provided to the input of a digital oscilloscope. The signal is processed with a digital high pass filter and displayed to a user or recorded as may be desired.

It is important to note in FIG. 2 that the spectral filter placed in the modulator output preferentially removes the original gate optical frequency, leaving only the signal frequency. As a result, the open eye pattern demonstrates that the bit pattern on the gate mode has been transferred via the all-optical modulator to the signal mode. This type of operation is likely to be of use in the construction of future all-optical networks.

Characterization with Sinusoidal Excitation

To better understand the device response with lower gate powers, and to characterize the frequency dependence of the mechanism, testing was performed with a gate having a sinusoidal intensity modulation. A lock-in amplifier was also used. FIG. 3 shows typical results. In the measurements described, an single-photon absorption modulator achieved a response to the gate of 0.6 degree/mW. We used a frequency of 32 MHz, a gate wavelength of 1541 mm, and a signal wavelength of 1550.8 nm to test single-photon absorption modulators with varying arm lengths, frequencies and wavelengths. The extinction on the sinusoidal modulation was around 20%.

We measured the dependence of the device response on the signal wavelength. As shown in FIG. 3, the maximum response occurred at bias points of 90 or −90 degrees. These bias points represent the locations of the maximum response for a phase-modulation-based effect. This is compelling evidence that the single-photon absorption mechanism primarily causes phase modulation, not intensity modulation. Changing the gate wavelength did not lead to a notable change in device performance. This response is as expected, since at no point does the gate mode interfere with itself, nor should the phase of the gate mode affect the signal mode.

FIG. 3 is a diagram showing the results of lock-in amplifier assisted measurements. FIG. 3( a) is a graph showing the device response, in peak to peak intensity of modulated signal, as a function of gate power. The extinction on the gate is about 20%. The signal power reported is the projected average power in the silicon waveguide just before the initial Mach-Zehnder y-junction. FIG. 3( b) is a graph showing the device response in similar circumstances as a function of varying signal power. In both cases, a linear relation is observed. FIG. 3( c) is a graph showing the device response as a function of signal wavelength, as well as the passive transmission through the single-photon absorption modulator of the signal alone. Maximum response is found at 90 and 270 degree bias points, which have 50% signal mode transmission, corresponding to a phase modulation mechanism.

The data shown in FIG. 3 also provide insight into the quantum mechanism behind the all-optical modulation. In particular, a single photon mechanism is suggested. If the modulation were based on a two-photon process, then the increase in peak-to-peak output intensity should be quadratic in increasing gate powers. Note that as in the pseudo-random bit sequence experiments, a filter has been used to selectively view only the signal optical mode.

Frequency Dependence

The frequency of the sinusoidal gate modulation was varied, and further data was taken to determine the bandwidth of the all-optical modulation mechanism. As before, a lock-in amplifier was used along with a filter to isolate the signal optical mode on the device output. As shown in FIG. 4, data were taken at speeds from 30 KHz to 50 MHz. The device response was fairly flat from 1 to 50 MHz. The peak to peak intensity modulation measured on the output gate mode is shown as a function of gate modulation frequency, for a constant gate power. We surmise that the higher response at slower speeds is likely due to heating.

Higher speed data were taken by means of a lightwave component analyzer (LCA). In this case, an Agilent 8703B (available from Agilent Technologies, Inc, 5301 Stevens Creek Blvd, Santa Clara Calif. 95051) was used. As before a spectral filter was used to remove the gate mode from the output of the device, ensuring that the LCA measured only the intensity modulation due to the all-optical modulation effect. FIG. 5 shows the results that were obtained. There is clearly a substantial falloff in the device response as the modulation speed approaches 1 GHz. As will be discussed shortly, this is evidence of free-carrier mediation.

FIG. 5 is a graphical diagram illustrating the high speed frequency response of a single photon absorption modulator device. As before, the peak to peak intensity modulation measured on the output gate mode is shown as a function of gate modulation frequency, for a constant gate power. A control is shown in which the signal power was set to zero and the gate to around 15.5 mW, confirming that the gate mode has been filtered out fully.

Testing with a Pulse Stream

To measure the polarity of the mechanism, as well as to observe the performance at slightly higher levels of gate power, measurements were performed in which the gate was pulsed at a repetition rate of 30 MHz and a pulse width of 8 ns. The wavelength was chosen to utilize the arm length difference to bias the Mach-Zehnder offset θ as it appears in Eqn. (1) in the theoretical discussion section. It was found that the polarity of the phase shift was negative; that is, ∂φ/∂P_(gate) is negative. Device response of approximately −0.9 degree/mW was observed at a peak gate power of 33.5 mW, for a total phase shift of −30 degrees. A negative index shift is consistent with the effects free carriers, but not the Kerr effect or heating. Details are shown in FIG. 6.

FIG. 6 is a diagram illustrating in graphical form the results of pulsed measurements observed using a single photon absorption modulator. The projected time domain trace of the gate power is shown in the lowermost curve. The data has been normalized so that the known insertion loss due to the grating couplers has been removed. As a result, the power shown in the upper two traces is the propagating power in the modulation waveguide. The output signal powers are shown for two different bias points, giving two different angular bias points of −82 degrees (uppermost curve) and 113 degrees (middle curve), respectively. In this case, the power reported is the projected power in the silicon waveguide past the final Mach-Zehnder y-junction.

Calculation of Waveguide Surface to Volume Ratio

We have indicated that the devices fabricated in silicon described herein use surface-state absorption involving defects in the surfaces of the waveguide. There are other possible mechanisms for causing single-photon absorption to occur, including damage to the volume of the silicon waveguide, for example by implanting ions in the silicon. Therefore, it is reasonable to consider the surface to volume ratio of waveguides as described herein, in order to evaluate which waveguides might be better suited to operate in the single-photon absorption mode using surface defects. In order to calculate the relative surface to volume ratios of various configurations of waveguides, we will take a 1 micron length of the waveguide as a unit of length, because the other dimensions that we consider are given in microns as well. The surface area of the 1 μm length of waveguide is computed as the sum of the products of the 1 μm length multiplied by each of the surface dimensions (top width, bottom width, and each of the side depths or thicknesses), all measured in units of microns. The volume of the 1 μm length of the waveguide is the product of the 1 μm length times the waveguide width times the waveguide depth (or thickness), all measured in units of microns. Table 1 lists several prior art wave guide dimensions as well as the dimensions used in an embodiment of a single photon absorption device described herein. The waveguide cross sections are taken to be rectangular. As may be seen from Table 1, the surface to volume ratio for the waveguides we describe is substantially greater than either of the surface to volume ratios of the prior art waveguides of others. This is a consequence of the smaller dimensions we use for the waveguides described herein, as compared to the prior art waveguides. The surface to volume ratio we used in this work is computed to be 24 μm⁻¹. The results in Table 1 suggest that a surface to volume ratio is the range of 18 μm⁻¹ or more would appear to be useful in providing silicon waveguide-based systems that exhibit single photon absorption. We expect that surface to volume ratios of 19 μm⁻¹, 20 μm⁻¹, 21 μm⁻¹, 22 μm⁻¹ and 23 μm⁻¹ would also be effective. Various waveguide dimensions, as shown in Table 1 as hypothesized examples, can provide such surface to volume ratios. Based on this analysis, we expect that waveguides of even smaller dimensions will have still larger surface to volume ratios that 24 μm⁻¹, and should be expected to exhibit single photon absorption behavior as well. This analysis does not take into consideration that oxides are known to passivate surface defects in silicon, and therefore, the surface of the waveguide contacting the silicon oxide layer might be omitted in other calculations.

TABLE 1 Surface/ Surface Volume Length Width Depth Area Volume Ratio Reference (μm) (μm) (μm) (μm)³ (μm)³ (μm)⁻¹ Prior art 1 2 0.9 5.8 1.8 3.222222 Prior art 1 0.45 0.25 1.4 0.1125 12.44444 This work 1 0.5 0.1 1.2 0.05 24 This work- 1 1 0.125 2.25 0.125 18 hypothesis This work- 1 0.6 0.125 1.45 0.075 19.33333 hypothesis This work- 1 0.5 0.125 1.25 0.0625 20 hypothesis This work- 1 0.5 0.11 1.22 0.055 22.18182 hypothesis This work- 1 0.1 0.1 0.4 0.01 40 hypothesis Construction of an Operational Device

We will now describe how an operational device, such as a single “optical transistor,” can be constructed. Using the devices described with regard to FIG. 2 as a starting point, one can substitute a digital signal generator, such as an electrical digital signal generator, for the electrical pseudorandom binary sequence generator, so that the signal applied to the gate is a signal having a desired value. One can apply an optical signal to be modulated as an input signal to the input port of the device using a laser with an amplifier such an EDFA as may be required. One can detect the output at the output gate using a filter as may be required or desired, and using a photodetector that provides an electrical signal representative of the optical output signal. In building a more complex device, such as a logic gate, a memory element, or other optical signal processing circuitry, one can cascade more than one device, using, the output signal of one device as the input to a gate on another device, or using the output signal of one device as an input (possibly in combination with the output from a second device) as input to the input port of another device. Circuits can be constructed by analogy to known digital or analog circuit designs that use electrical transistors as elements, and which operate using similar logical rules and procedures. By analogy to electrical circuits, in which reference voltages such as various supply voltages and/or ground potential are commonly used, optical systems can use optical signals having predefined characteristics, such as defined frequency and intensity (or power), as reference signals. As will be appreciated, all components that require electrical power to operate (e.g., laser sources, electrical signal generators, an electrical detector or analyzer configured to receive the output of a photodetector, and so forth) can be powered using conventional power supplies, which draw power from conventional electrical power distribution systems (e.g., wall sockets), or from portable electric power supplies, such as batteries. While the structure described herein is a Mach-Zehnder configuration, other configurations for modulators using waveguides are also known, and it is expected that such other configurations may be equally useful in constructing optical modulators that operate in the single-photon absorption regime. Examples of other configurations include ring resonators and photonic crystal or periodic Bragg-grating-based waveguide structures. Optical resonator configurations such as these have the potential to enhance the device response to a given number of single-photon absorption-generated free carriers.

The lack of a low power optical switch, the optical equivalent of a transistor, is one reason that an all-optical computer has not yet been developed. One might consider building an optical computation device using a plurality of single-photon absorption based switches. One approach would be to use the single-photon absorption switch to construct the optical equivalent of a flip-flop; that is, a device that has two stable states, which can encode a 1 or a 0. Another possibility is to use single-photon absorption switching regions on two arms of a Mach-Zehnder interferometer, which could be used to construct an AND gate. Because these elements would be relatively low in power for a well-designed single-photon absorption switch, it is expected that one could construct many of them in close proximity, allowing the creation of extremely fast and powerful computation engines.

In general, a logic gate or logic element built using the single-photon absorption based methods described would implement a logic function as follows. One would define a state of the input optical signal and a state of the gate signal. The modulator configured to implement the desired logic function would provide at its output an optical signal that represents the desired logic function as a state of the output signal relative to the state of said input signal. For example, an inverter would provide an output signal having the inverted value of the input signal (e.g., input is “0” or “low” or “false”, output is “1” or “high” or “true”). As mentioned, using two input gates, one in each arm of a balanced Mach-Zehnder, might be used to sum two signal, or with the addition of an inverter, or of a gate that introduces a 180 degree modulation, to obtain the difference between two signals.

Distinguishing Single-Photon Absorption (SPA) Based Devices from Two-Photon Absorption (TPA) Devices

It is expected that it will be possible to distinguish by observation whether a device is based on a single-photon absorption or two-photon absorption mechanism, for example based on the device response to the gate optical mode strength. A device based on a single-photon absorption mechanism will be expected to continue to have a linear response to the gate optical mode strength, while the response from the two-photon absorption device will be quadratic. By way of example, consider the small-signal response for a device that is characterized in terms of 10 degrees of response for a certain amount of gate power. If the power is now reduced by a factor of 100, a single-photon absorption based device will now have a response of 0.1 degrees, but a two-photon absorption based device will now have a response of 0.001 degrees. As a practical matter, this means that for smaller powers the single-photon absorption device will continue to function, but at a certain point the two-photon absorption device will practically cease to respond to the gate optical mode. At very high powers, for particularly efficient devices, another data point can be used. The two-photon absorption based device will always have a quantum efficiency in terms of switching electrons and holes that is around one-half that of the single-photon absorption based device. While gain due to avalanche processes or some other intermediate step may obscure this somewhat, it should still be possible to determine which mechanism is operative based on the operational limitations and/or response as a function of input power on device performance.

All-Silicon Photodetector

Silicon is an extremely attractive material platform for integrated optics at telecommunications wavelengths, particularly for integration with CMOS circuits. Low loss waveguides, high-Q resonators, high speed modulators, efficient couplers, and optically pumped lasers have all been demonstrated. Developing detectors and electrically pumped lasers at telecom wavelengths are the two main technological hurdles before silicon can become a comprehensive platform for integrated optics.

Silicon's bandgap of 1.12 eV makes it challenging to build a silicon-based detector in the near infrared. Silicon has minimal absorption of photons in this regime, at least in the bulk, defect-free case. Two-photon absorption can potentially be used to circumvent this limit and build a detector, but for practical power levels the efficiency is poor. Approaches to detection have typically relied upon bonded III-V materials, on integrating Germanium or use of SiGe alloy materials, or more recently, through volume defect creation via ion implantation.

A photocurrent has also been observed in undamaged silicon waveguides, and has been attributed to an effect from the surface of the waveguide, though quantum efficiencies of only 0.24% were shown. Here we show a single photon photodetector based on surface states of a SOI ridge waveguide. Because of the large modal overlap with the surface of the waveguide for our particular geometry, photons are more efficiently absorbed, and a quantum efficiency of 2.8% is obtained. It is expected that there can be many different mechanisms that might provide single photon absorption in addition to surface state effects and bulk defects such as those created by ion implantation, including such mechanisms as different types of damage to the silicon material, use of different ion implant species, and use of other kinds of materials such as germanium, tin, and alloys thereof, III/V materials and alloys thereof banded materials, epitaxially grown materials, deposited materials and others. The electrical, optical and crystallographic characteristics of such materials can be modified or adjusted by incorporating dopants therein.

It is also expected that one can add one or more semiconductor junctions (such as a p-n junction, a p-i-n junction, or other types of junctions) or an electrical gate to such types of structures, in order to provide a device that can also provide electrical amplification.

It is anticipated that resonant enhancement may also be utilized in order to enhance the performance of these devices, either by substituting a resonator for the Mach-Zehnder interferometer (in order to create a resonantly enhanced device, rather than one relying on raveling waves), or by combining a resonator with the interferometer.

Waveguide Geometry

It is well known that defect states can form at the edge of a crystalline semiconductor. Such defects are known to contribute substantially to the optical losses of silicon waveguides. Most low-loss silicon waveguide geometries involve fairly large silicon waveguides, on the scale of at least 0.450 μm×0.250 μm, and often more than 2 μm×0.9 μm. We instead use a geometry of 0.5 μm×0.1 μm, obtaining losses of around 5 dB/cm. FIG. 1( a) through FIG. 1( e) show diagrams of the waveguide geometry used, as well as information on the mode distribution and SEM micrographs of the waveguide.

In FIG. 1( a) the modal pattern for the TE-1 mode is illustrated overlaid on a drawing of the waveguide, Contours are drawn in |E| in increments of 10% of max value. For a propagating power of 1 W, the peak electric field will be about 10⁸ V/m. In FIG. 1( d) there is shown an SEM micrograph of a detector device of type B, as defined herein below. In FIG. 1( e) there is shown another SEM micrograph of a device of type B. A ridge waveguide is contacted by a series of tiny, conductive arms. The optical mode is tightly confined to the ridge waveguide, and does not appreciably touch the metal pads or the surrounding silicon layer.

Polymers, oxides or air can be used as claddings. Loss comes primarily from three effects: scattering from residual lithographic roughness, absorption from surface states, and absorption from the bulk silicon. These surface states exist precisely where the optical mode field of the waveguide is at its peak intensity, because the electric field is normal to a high dielectric discontinuity. A grating coupler was used to couple light from a standard fiber optic mode pattern into the ridge waveguide.

Device Fabrication

Devices were fabricated in electronics-grade SOI from Soitec, doped at around 10¹⁵ dopants (Boron)/cm³. No implant or irradiation is performed on the silicon material. The starting material was oxidation thinned to about 110 nm by dry oxidation, singulated into small chips, and patterned using electron-beam lithography on a 100 kV electron beam writer using HSQ resist. The samples were etched with chlorine in an inductively coupled plasma etcher. After removing the residual resist and native oxide, photolithography and evaporation were used to define and deposit aluminum electrodes. No cladding layer was deposited for all devices.

Electrical Measurements

Device Layout

The most straightforward way of observing the photocurrent effect is to electrically contact the optically active area, and apply a bias voltage. The surface state absorption will change the conductivity of the device, and this will result in a photocurrent. Two device types were studied: in type A, the grating couplers were contacted directly with large silicon arms. In type B, small silicon arms can be used to form contacts to the waveguide directly; only around 0.2 dB of optical insertion loss is endured from 0.07 μm silicon arms. These two device layouts are shown in FIG. 8( a) and FIG. 8( b), FIG. 8( a) is a diagram illustrating a detector of type A. The photoconduction region comprises the loop of the ridge waveguide, while the grating couplers are connected to the metal electrodes. FIG. 8( b) is a diagram illustrating a detector of type B. Here the photoconduction region is the intersection of the conduction arms and the waveguide. FIG. 8( c) is a diagram illustrating an equivalent circuit, with Rp the photoconductive resistor, and Rm the resistance of the measuring apparatus. The diodes present in the circuit are due to the metal-semiconductor interface. Device type A had a length of 0.4 mm, while device type B had a length of 1.5 mm, and 40 contacting arms. In both cases, light flows into one grating coupler, through the electrically contacted ridge waveguide, and finally out through another grating coupler, SEM micrographs of devices of type B are shown in FIG. 1( d) and FIG. 1( e).

It is important to note that that the optical field is separated by tens of microns from the region where the metal pads touch the silicon for both devices. No propagating mode is supported along the tiny conductive arms, and the insertion loss due to each arm is minimal, as confirmed by both simulation and, measurement. This is significant, as a Schottky harrier can result in a photocurrent for near infrared radiation on the basis of internal photoemission. If the optical mode did touch the metal-silicon barrier, this could be a possible source of the photocurrent that is observed, but the geometry used excludes this possibility.

DC I-V Measurements

The devices were first characterized by coupling a continuous wave laser at 1575 nm through the input coupler, and measuring the DC current that flowed in response to a series of voltages. Several devices were studied, a device of type A (A1) and of two of type B (B1, B2), FIG. 8( d) is a diagram illustrating the entire experimental setup. For DC I-V curves, the lockin is replaced with a picoammeter.

The measured IV curves for the two devices studied are shown in FIG. 9( a) and FIG. 9( b). FIG. 9( a) is a graph showing DC I-V curves for device A1, FIG. 9( b) is a graph showing DC I-V curves for device B1. The propagating power shown is the power in the waveguide after the loss due to the grating coupler and other parts of the optical test apparatus. For the data shown, input light at 1575 nm was used. In the case with no incident radiation, the data show the effect of the rectifying contact created by an aluminum electrode on top of lightly p-type silicon. This rectification limits device performance, and decreases the quantum efficiency as a result. Ohmic contacts can easily be formed in future devices with contact doping and annealing. By constraining a heavy p-doping to the pad region only, it should be possible to minimize rectifying behavior, while leave optical behavior unchanged.

The power of the test laser used, as well as the propagating laser power in each device is labeled. Slightly different optical paths used in each series of tests result in different levels of propagating power for the same laser power. Current in nA is plotted against voltage in V. Note that FIG. 9( a) shows significantly more rectifying behavior, because there are only two small contacts to the waveguide in this case, while in FIG. 9( b) there were 40 contacts to the guide FIG. 9( c) is a graph showing the peak to peak output photocurrent of device B2 as a function of frequency. There is minimal change in performance from DC to approximately 60 MHz, where testing was stopped due to limitations of the noise environment where the devices were being tested.

When light is incident on the waveguide, a photocurrent is observed when the device is under bias. The current across the device increases along with the laser intensity. For high power, the device response is not linear in laser power, but shows sub-linear behavior. This is attributable to the rectifying contact in series with the photoconductor. In the best case for device A1, the direct current responsivity at an 11 V bias is 0.1 mA/W, while for device B1 the responsivity in the best case is 1.5 mA/W, corresponding to a quantum efficiency of 0.12%. One of the reasons that device B1's performance is superior is that it has far more semiconductor-metal contacts, allowing more current to flow. It is also possible that the tiny arms connecting the waveguide to the electrodes present additional surface states where optical absorption can occur. Finally, it can be shown that excess electrons and holes can only be cleared from a small portion, around 10 μm of waveguide, from device A1. Therefore, devices of type A had much smaller effective lengths than devices of type B. Device B1 was also measured at lower input powers, where performance improved, and the device response became linear.

Device B2 was measured at higher powers by using an Erbium-Doped Fiber Amplifier (EDFA), with around 10 mW of power propagating in the waveguide. A lockin was also used to precisely measure the output current, as shown in FIG. 9( c). A responsivity of 12 mA/W at 7 MHz was obtained, with nearly flat performance in frequency. From 1 KHz to 60 MHz, the responsivity changes by only a factor of 3. The slight nonuniformity seen is likely due to electronic parasitics, and perhaps the testing environment. At lower frequencies, the larger increase in response from DC to 1 kHz is likely due to some sort of parasitic capacitive coupling in the metal-semiconductor contact. We believe that the device would continue to perform past 60 MHz, however environmental noise precludes measurements at these higher speeds. The decreased responsivity compared to the results observed for B1 is believed to be due to the large amounts of power used in testing at high frequencies. It should be pointed out that the photocurrent from the two controls listed in FIG. 9( c) dramatically increases at higher frequencies. This is believed to be a result of increasing radiative cross talk between the lockin and the function generator at higher frequencies.

It is believed that the effect involved is extremely fast, and it is likely that with proper supporting electronics, a detector with speeds in the gigahertz could be constructed. The ultimate limit of the device is believed to be the time it takes to sweep free carriers across a micron scale silicon device. This speed can easily exceed 10 GHz. It is believed that defect absorption centers in silicon can have extremely rapid response times. These measurements were also performed with similar devices that were clad in PMMA, rather than exposed to the atmosphere. A similar level of photocurrent was observed, with very similar performance.

Low Intensity Measurements

It was observed that for larger optical intensities, the dependence of the photocurrent was sublinear, in that doubling the optical input intensity resulted in less than a twofold increase in photocurrent. To better understand the photocurrent process, as well as to estimate the performance in the absence of a rectifying contact, a Princeton Research 5210 lock-in amplifier was used to characterize the photocurrent as a function of optical power. A time constant of 3 s was used for an excitation frequency of 1 KHz, and an input wavelength of 1575 nm. A lithium niobate optical modulator was used to impose a sinusoidal variation on the input intensity, producing a sinusoidally varying photocurrent. The incoming intensity wave is chosen to have nearly full extinction at its lowest point, implying the average power is roughly half of the peak to peak power swing. FIG. 10( a) shows the photocurrent as a function of power for several bias voltages. These measurements were taken for device B1. In FIG. 10( a), the peak to peak photocurrent in nA is reported as well as the peak to peak optical power in the waveguide in mW. The response that would be observed with a perfectly linear 1.5 mA/W and 36 mA/W detector are also shown. The responsivity is seen to continue to improve for lower optical powers, eventually becoming linear. This effect is readily explained by the presence of a reverse biased diode, the character of which is seen in the DC curves in FIG. 9( a) and FIG. 9( b). In the best case, a linear responsivity of 36 mA/W is obtained, corresponding to a quantum efficiency of 2.8% for an 11 V bias. It is worth noting that for higher optical power levels, the responsivity observed with DC measurements is approached. FIG. 10( b) is a graph showing the photocurrent in peak to peak nA of the device for a 11 V bias voltages and several peak to peak laser powers as a function of frequency up to 1 MHz. In both FIG. 10( a) and FIG. 10( b), a logarithmic plot has been used.

We believe that in the future, more optimal electrode and device geometries will enable the device to demonstrate linear responsivity at much higher optical powers. This could be achieved on the basis of doping the silicon to increase the conductivity, and by pad implants to decrease the contact resistance.

Optoelectronic Transistor

it is recognized that a significant accomplishment in nonlinear optics would be to provide full all-optical logic. For a system to be practical, it preferably should be compact, scaled to fit on a substrate sized comparably to a semiconductor chip, operate using relatively low intensity continuous-wave optical power, and permit each element to be operated or pumped by a different element in an array of diode lasers, in a preferred embodiment, the optical pumps will be situated on-chip. In some embodiments, the pumps can be expected to comprise wafer-bonded devices comprising III/V compound materials or their alloys.

It is expected that it will be possible to build a free-carrier based all-optical logic platform for 10-100 GHz applications. Some applications that can be envisioned include devices and systems that provide optical signal amplification, wavelength conversion to or from arbitrary optical wavelengths, regeneration, clock (or frequency) generation in the optical domain, analog signal processing, high-bandwidth packet multiplexing and demultiplexing, delay lines providing arbitrary delays, optical memory, and many other things that are done today with electronic transistors, except that the functions can be done in an all-optical system.

By employing the single-photon absorption effects which have been described herein that are operative in silicon waveguides, one can combine optical inputs and outputs with electronic gain to create an optoelectronic transistor. It is understood that electronic junctions are very good for providing gain, especially through avalanche processes. As one example, it is possible to make integrated photodetectors at 1550 nm with these effects.

FIG. 12 is a diagram in perspective illustrating the structure and operation of a portion of an all-optical transistor. The all-optical transistor can be expected to operate as follows. Light comes in on a ‘gate’ waveguide and a ‘signal’ waveguide. Photons from the gate waveguide are absorbed through a single-photon process. A lateral bias provided by applying a voltage across electrodes on the silicon device sweeps the carriers toward the signal guide. These carriers are multiplied through an avalanche process in the semiconductor. A phase shift is created by this large population of carriers in the signal guide. This phase shift is translated into amplitude modulation by utilizing a Mach-Zehnder interferometer.

FIG. 13( a) is a diagram 1300 in side view of a portion of an illustrative all-optical transistor. In FIG. 13( a), a structure similar to that of FIG. 12 is described. In the illustrative embodiment of FIG. 13( a), a silicon layer 1310 is provided, which can be, for example a silicon layer of a silicon on insulator (SOI) wafer fabricated on a silicon wafer substrate. The silicon layer 1300 is doped with conventional dopant atoms, such as those from column III of the periodic table (to provide holes, or p-type material) or those from column V of the periodic table (to provide electrons, or n-type material). Silicon optical waveguides 1320 and 1330 are shown end on. In one embodiment, the two waveguides are separated by a distance of 0.7 μm, and have a thickness or height of approximately 200 nm. The waveguide 1320 can be the gate waveguide, adjacent to which the silicon layer 1310 may have been deliberately damaged, for example by ion implantation with silicon ions. The damaged silicon layer may absorb a photon from the gate waveguide 1320. Under the application of an electrical field across the silicon layer 1310, as illustrated by the connection of a positive voltage V+ on one side of the layer 1310 and the connection of the other side of the layer 1310 to ground potential, the carriers generated are accelerated, and charge multiplication by an avalanche process is caused to occur. The charges so generated affect the optical properties of the signal waveguide 1320, which can be one leg of a Mach-Zehnder interferometer (with the other leg not illustrated in FIG. 13( a).

FIG. 13( b) is a diagram in plan view of a portion of an illustrative all-optical transistor, such as that of FIG. 12. FIG. 13( b) shows the relative placement of the gate waveguide, the signal waveguide, the implant region, and the positions where the voltage signals are expected to be applied to the device. The implant region comprises one or more implanted ionic species, such that the desired (or optimal) p/n distribution will depend on the quantum mechanism operative in the junction region. For comparison, a line representing a distance of 0.7 μm is also shown.

FIG. 13( c) is a schematic diagram illustrating the logical layout of an illustrative all-optical transistor. In FIG. 13( c) the signal waveguide is one arm of a Mach-Zehnder interferometer, and the gate waveguide provides an input signal that modulates the optical index (e.g. causes a Δn) of the signal waveguide.

FIG. 14 is a schematic plan diagram showing the layout of an all-optical transistor that comprises a Mach-Zehnder interferometer geometry, in which one arm of the Mach-Zehnder is the signal waveguide of FIG. 13( b).

FIG. 15 is a schematic plan diagram showing the layout of an all-optical transistor that comprises a ring resonator geometry.

The all-optical Mach-Zehnder switch can be envisioned for use as an amplifier, an all-optical logic element, or a nonlinear all-optical signal processing element that could operate at 100 GHz. The operation of the device could be tuned by controlling the DC bias.

It is expected that with a resonantly enhanced device, very low (for example mW scale) gate power could be realized, according to the relation:

$\frac{\partial\lambda}{\partial P_{gate}} \sim {\frac{\lambda_{0}}{n}\frac{\partial n}{\partial P_{gate}}}$

Some estimates of the performance of such devices can be presented. We have a hole saturation velocity of ˜8×10⁶ cm/s. For a distance of approximately 0.7 μm, it takes only about 9 ps for the charge carriers to make the transit at the saturation velocity. This suggests that a bandwidth in excess of 100 GHz is possible.

We can also consider the responsivity in radians/mW, which will determine how much power will be needed for a desired phase shift, such as 180 degrees or π radians. In radians, Pπ=π/Responsivity, which corresponds to turning the switch totally on or off. This is discussed further elsewhere in this document.

Theoretical Discussion

Mach-Zehnder Device

Although the theoretical description given herein is thought to be correct, the operation of the devices described and claimed herein does not depend upon the accuracy or validity of the theoretical description. That is, later theoretical developments that may explain the observed results on a basis different from the theory presented herein will not detract from the inventions described herein.

Evidence of Single-Photon Absorption Mechanism

There are a number of possible mechanisms by which one near-infrared optical mode can induce a phase shift in a second near-infrared optical mode in a silicon waveguide. These consist of thermal interaction, the Kerr effect, or free-carrier dispersion, which can be due to two-photon absorption, or in the case of our all-optical modulator, single-photon absorption. Whatever the mechanism, it is possible to express the all-optical interaction as follows: P _(out) =C(1+cos(γ((1+b cos(ωτ))P _(gate))^(α)+θ))P _(signal) ^(β)  Eqn. (1) where we have assumed that the gate signal has the form (1+b cos(ωτ))P _(gate)  Eqn. (2)

In Eqn. (1), P_(out) is the output power as a function of time of the filtered signal optical mode, and γ and C are proportionality constants characteristic of the interaction. We have also assumed the signal is at a speed far below the bandwidth limit of the relevant effect. If the modulator is biased at the point of maximal response, that is, choosing θ to be ±π/2, then the peak to peak intensity modulation on the output will be proportional to R∝2γb ^(α) P _(gate) ^(A) P _(signal) ^(β)  Eqn. (3) if the modulation effect produces much less than π/2 radians of phase shift. In all of the experiments described herein, except where explicitly noted otherwise, the modulator bias was always set to this value. It was easy to do this, because far less than π/2 radians of shift was achieved in even the highest gate power cases, and so this amounted to simply tuning the signal wavelength to maximize the device response. In the case of a linear modulation mechanism, that is, where α and β are both 1, γ can be identified as ∂φ/∂P_(gate).

Other than single photon absorption, the mechanisms that exhibit this power law are two photon absorption-based absorption modulation, the Kerr effect, and Raman scattering. In all three cases, however, there exists no bandwidth limitation near 1 GHz. Of these effects, a phase shift would be observed for only the Kerr effect. Moreover, Raman scattering must involve wavelengths separated by the Raman shift—15.6 THz in silicon—which is clearly irrelevant to the current result. As shown in table 1, the observed gate response is also more than an order of magnitude larger than what would be achievable with any mechanism other than single photon absorption, even in an idealized case. Single-photon absorption is the only mechanism consistent with the observed data.

The lower gate responses of other modulation mechanisms result in typical effective P_(π) values from 5 Watts, to 100 Watts. The high modulation power requires most other devices to be operated in pulsed mode. Even when resonant enhancement is used to greatly lower the external power required, the internal circulating power must still be quite high. By contrast, our single photon absorption based device can be projected to achieve complete extinction at around 180 mW P_(π), over an order of magnitude lower than is achieved with existing mechanisms. No further resonant enhancement would be required. Furthermore, silicon nano-waveguides can carry bit streams with average power levels in the tens and hundreds of milli-Watts. However, multi-Watt average power levels will rapidly damage the guides.

Table 2 provides a list of all-optical modulation mechanisms in silicon waveguides. For each mechanism, the two exponents from Eqn. (3) are shown. For phase modulation mechanisms, the theoretical gate response for the silicon ridge waveguide studied is given, for powers near 5 mW. Also shown is the response achieved in an actual realization. The device response for single-photon absorption, representing the devices described herein, is for a 1.2 cm Mach-Zehnder modulator, with gate power near 5 mW. In writing the exponents, we have assumed that the shift in refractive index is linearly proportional to the number of free carriers, which is approximately valid in the relevant optical power regime. We have also assumed that the bandwidth of the single photon absorption mechanism is limited by the free-carrier lifetime. The discrepancy between the theoretical and observed performance is due in part to the larger waveguides used in typical realizations, compared to the 0.5×0.1 μm ridge used here, as well as effective lengths that may be less than 1.2 cm. Since carrier injection from TPA (II) is not linear in the input signal, it is impossible to quantify a comparable gate response. The estimate of the single-photon absorption modulator's theoretically best performance assumes that an electron and hole are created with each absorbed photon. One of the works referred to (Q, Xu, and NM. Lipson) utilizes resonant enhancement. For clarity of comparison, the projected performance in a non-resonant Mach-Zehnder modulator is shown. Note that the achieved performance in our device exceeds the theoretical limit for the Kerr effect as well as carrier-injected TPA (I).

TABLE 2 ALL-OPTICAL MODULATION MECHANISMS IN SILICON WAVEGUIDES Theoretical Typical maximum effective gate gate response response near 5 mW near 5 mW Effect α β Bandwidth (degrees/mW) (degrees/mW) Electro-optic N/A 1 Free carrier limited N/A N/A Modulation Carrier injection 2 1 Free carrier limited 0.1 7 × 10⁻⁵[*] from TPA (I) Carrier injection 1 2 Free carrier limited N/A N/A from TPA (II) TPA based absorption 1 1 Ultrafast N/A N/A Kerr Effect 1 1 Ultrafast 0.14 0.0018[**] Stimulated Raman 1 1 Raman gain bandwidth, N/A N/A Scattering 100 GHz Single-photon 1 1 Apparently free carrier 32 1 absorption (This work) limited [*]Q, Xu, and M. Lipson, “All-optical logic based on silicon micro-ring resonators,” Optics Express, vol. 15, pp. 924-929, 2007. [**]Ö. Boyraz, P. Koonath, V. Raghunathan, and B. Jalali, “All optical switching and continuum generation in silicon waveguides,” Optics Express, vol. 12, pp. 4094-4102, 2004. Free-Carrier Model of Modulation Effect

If the hypothetical model of the creation of both an electron and hole is used, the shift in electron-hole concentration can be written as:

$\begin{matrix} {{\Delta\; N} = {\tau\frac{\sigma}{A}P}} & {{Eqn}.\mspace{14mu}(4)} \end{matrix}$

Here P is the gate power, and A is the waveguide area. We introduce σ, the absorption density, which determines the number of photons absorbed per unit length in the waveguide for a given amount of power. The absorption density has units of cm⁻¹s⁻¹mW⁻¹. The parasitic loss from absorption will be: α=hvσ  Eqn. (5) where h is Planck's constant, and v is the relevant photon frequency, around 193 THz.

We could then express the device response using the following free-carrier dispersion relations

$\begin{matrix} {\frac{\partial\phi}{\partial P_{gate}} = {{- \frac{\partial}{\partial P_{gate}}}{\int_{0}^{L}{{\mathbb{d}{z\left( {{\left( {{P_{gate}{\exp\left( {{- \alpha}\; z} \right)}\frac{\tau\sigma}{A}} + p_{0}} \right)^{8}8.5 \times 10^{- 18}} + {\left( {{P_{gate}{\exp\left( {{- \alpha}\; z} \right)}\frac{\tau\sigma}{A}} + n_{0}} \right)8.8 \times 10^{- 22}}} \right)}}\frac{\partial n_{eff}}{\partial n_{bulk}}*k_{c}}}}} & {{Eqn}.\mspace{14mu}(6)} \end{matrix}$

A similar expression would be used if only a free hole were created, and would in fact provide comparable results, since free holes have a larger influence on the induced refractive index shift. Here, A is the waveguide area, p₀ and n₀ are equilibrium hole and electron concentrations, P_(gate) is the gate power, τ the recombination lifetime, σ is the absorption density, and α is the optical attenuation coefficient. Eqn, (6) will be nearly linear in the regime for which data were taken.

Eqn. (6) implicitly assumes that the operation speed is much lower then the bandwidth limitation. As is shown below, as the operation speed approaches the recombination lifetime, the device response in the form of peak-to-peak intensity of the modulated signal has the following behavior:

$\begin{matrix} {R \propto \frac{1}{\sqrt{1 + ({\omega\tau})^{2}}}} & {{Eqn}.\mspace{14mu}(7)} \end{matrix}$ where ω is the excitation frequency, and τ is the minority-carrier lifetime. As a result of Eqn. (1), the device response will have a 70% reduction at around f=½τ. This occurs near 1 GHz, suggesting a minority carrier lifetime of around 0.5 ns, similar to values reported for similar SOI ridge waveguides. This provides further evidence that the modulation effect is free-carrier mediated. Calculating Surface-State Absorption Density

Eqn. (6) can be solved for the observed phase shift once the recombination lifetime is known. In the case of the highest performing device, which achieved around 16.6 degrees for 15.5 mW of gate power, the absorption density can be calculated as 1.4×10¹⁴ cm⁻¹s⁻¹mW⁻¹. This implies a waveguide loss due to the single photon absorption mechanism of only 0.1 dB/cm, which is a small fraction of the waveguide loss seen. Typical excess carrier concentrations under testing conditions with this density are on the order of 7×10¹⁴ cm⁻³ for 5 mW of gate power.

Geis et al. measured the absorption density directly through a photocurrent measurement. A responsivity of 0.8 A/W was demonstrated in a 0.25 mm device, corresponding to a quantum efficiency of 64%. That implies the optical loss due solely to absorption would be at least 178 dB/cm, and from Eqn. (3) the absorption density can be estimated as 3.1×10¹⁷ cm⁻¹s⁻¹mW⁻¹.

Ultimate Performance Limitations

A complete explanation of the single photon absorption mechanism likely will require further work will be required to understand the single-photon absorption mechanism. One possibility is that an electron is excited into the conduction band from the valence band. In another possibility, the electron is knocked into a defect state, creating a free hole. As holes have a disproportionately large effect on the refractive index shift in silicon, the creation of a free hole would adequately explain the observed behavior.

We can estimate the ultimate performance limits if we hypothesize that both an electron and hole are created for each absorbed photon. The free-carrier lifetime would determine the ultimate modulation speed. Approaches have been developed to lower this lifetime and frequencies exceeding 5 GBit/sec should be possible. It is also possible to decrease the gate power required for modulation by increasing the density of photon-absorption centers, at the expense of insertion loss. We estimate that, allowing 6 dB of additional loss from implantation-generated defect centers, a gate response of 32 degree/mW could be achieved at 0.5 GBit/sec, corresponding to a P_(π) around 6 mW; this would require around a factor of 100 increase in absorption density over the value currently obtained in our devices. Absorption center densities as much as three orders of magnitude higher than those in this work have been previously engineered, suggesting that these improvements are likely to be possible. FIG. 7 shows a graph of projected performance values for varying absorption densities.

FIG. 7 is a diagram that illustrates projected performance of single photon absorption devices for varying surface-state absorption densities. A response of nearly 32 degree/mW could be obtained for absorption densities a actor of 100 higher than the projected value in our device.

Derivation of Bandwidth Limit for Device

The response of the Mach-Zehnder near the 3 dB bias point of the modulator is proportional to the phase shift induced by the gate signal. This in turn is nearly proportional to the free-carrier density. As a result, the frequency response can be calculated based on the time-dependant equation for the excess carrier concentration:

$\begin{matrix} {\frac{\mathbb{d}N}{\mathbb{d}t} = {G - \frac{N}{\tau}}} & {{Eqn}.\mspace{14mu}(8)} \end{matrix}$ where G is the generation rate in cm⁻³s⁻¹. Under the experimental conditions of a gate with a sinusoidal intensity modulation, G(t) will have the form G₀(1+b cos(wt)). The solution for N in this case is:

$\begin{matrix} {{N(t)} = {G_{0}{\tau\left( {1 + {{Re}\left( \frac{\exp\left( {{\mathbb{i}\omega}\; t} \right)}{1 + {{\mathbb{i}\omega}\; t}} \right)}} \right)}}} & {{Eqn}.\mspace{14mu}(9)} \end{matrix}$ Since the responsivity is proportional to the AC amplitude of N(t), it will have a frequency dependence of (1+(ωτ)²)^(−1/2). All-Silicon Photodetector Analysis Exclusion of Heating as a Significant Mechanism

A substantial concern that one might have is that the effect seen is solely due to heating of the waveguide. This would mean that such a detector would not be expected function at high speeds, thereby limiting its utility. We note that the conductivity of silicon doped with Boron at 10¹⁵ l/cm³ should not increase until the temperature reaches 230 C or more. This is due to the fact that n_(i) for silicon does not approach 10¹⁵ l/cm³ until this temperature is attained, and thus the conductivity will be mainly due to dopant contributions. We have observed that increasing the temperature does not typically increase device current. We can then examine a device of type A, and note that for the effect to be explained solely by heating, the volume 20 μm³ would have to be heated, by around 200 C. The specific heat of silicon is 702 J kg⁻¹ K⁻¹, and the density is 2.320 g cm⁻³. This implies that 6.5 nJ would be required to heat the waveguide by this amount.

The device was seen to function, however, with no frequency rolloff to 0.5 MHz with 100 μW of input power. At this higher speed the total amount of energy that can be delivered optically in a single cycle is 2×10⁻¹⁰ J. Under such conditions, there would not be enough energy to heat the waveguide by the large amount needed. The waveguide used in device type A is identical to the waveguide used in our loss calibration structures. From this, as well as device type A's optical performance, the optical loss in the loop is around 5 dB/cm, implying that only around 4.5% of the energy could be absorbed by all of the loss mechanisms combined. Therefore, the amount of energy that could be expected, to participate in a heating mechanism is around 1/1000 of the amount that would be needed for the conductivity to change based on a thermal effect.

It is possible, though unlikely, that the structure could first be heated by the dark current induced by the bias voltage, and then reach a very hot temperature at which the heating response to the optical signal was more pronounced than this analysis would suggest. Experiments were performed in which the sample was heated to a number of temperatures greater than 25 C, and the photocurrent observed. Generally, the photocurrent decreased quickly as temperatures rose. In a typical instance, a temperature rise of 20 C decreased the photocurrent by a factor of 2. This suggests that if there is heating from the DC current, the optical response would actually decrease. It also suggests that the possibility of the waveguide being heated to 200 C and this being the source of the photocurrent is even more remote. One explanation for the decreasing photocurrent is the effect of temperature on minority carrier lifetimes, because the photogenerated electrons will not contribute to the photocurrent if they recombine before reaching an electrode. When cooled to 25 C, the original performance was restored on all devices tested.

Another experiment was performed that also suggests that heating cannot be the source of the effect. As is well known, thermal heating in a ring resonator results in significant shifts in the resonance peaks. This is due to the fact that silicon's index of refraction will change with temperature. The resonance peaks of a ring resonator on the same chip as the detector devices were scanned using several different optical power levels. The levels ranged from around 10 μW to 1 mW. A power of 1 mW is more than 10 times the power level used in most of the detection experiments. In all cases, a negligible shift in peak appearance or location was observed. However, healing the silicon chip by even 40 C shifted the location of the peaks dramatically. This suggests that if there is any optical heating in our experiments, it is likely to be extremely small, probably less than 0.1 C. This is orders of magnitude less heating than could possibly explain a shift in photocurrent.

Exclusion of Two-Photon Absorption as a Significant Mechanism

It is well known that silicon can absorb radiation based on two-photon absorption if illuminated with a sufficiently intense beam of optical radiation. Given that the dimensions of our waveguides are quite small, and the corresponding optical intensities are large, this might seem to be a candidate for the effect seen. However, two-photon absorption is a nonlinear process, with the number of photons absorbed depending on the square of the optical intensity. As a result, an effect based on two-photon absorption would be quadratic in input power—that is, the generated current would be quadratic in input intensity. This is inconsistent with the linear dependence of current with optical power that we have observed. Nevertheless, further experiments were performed to determine whether two-photon processes might be operative.

To determine the amount of two-photon absorption that could be present, calibration structures of two grating couplers and 300 μm of waveguide length were exposed to intense optical radiation beams by using an EDFA. The amount of optical radiation was varied from approximately 100 μW to 200 mW, and the transmission through the device was observed. Linear performance was observed until the illumination power reached 100 mW. The nonlinear loss can then be estimated to be at most approximately 250 dB cm⁻¹W⁻¹. Measurements with our detectors were performed with 10 μW of power or less, in devices of length 1000 μm. In such a situation, the nonlinear loss stated previously would imply only 0.00025 dB of loss, corresponding to an optical loss of 0.005%. This would provide an upper bound for the quantum efficiency to around 0.0025%, since for every two photons absorbed in this process, a single electron hole pair would be created. This value, however, is nearly a factor of 1000 less than the efficiency observed. Moreover, the nonlinear loss in silicon at high optical powers is believed not to be due solely to two-photon absorption, but mainly to free carriers that have been created by this process. As a result, the quantum efficiency that could be obtained by two-photon absorption is probably much lower than what was observed.

Mechanism and Identification of Surface States as Source of Photoconductivity

The precise mechanism of the photocurrent is unclear at present. One possibility is that electrons and holes are being created, despite the fact that the photon energy does not exceed the band gap energy of silicon. Another possibility is that only free holes are being created, and the surface state creates a mid-bandgap acceptor state.

If one accepts that the photocurrent is caused by the generation of an electron and a hole through absorption at a defect state, then the question of whether these defects are in bulk or are at the surfaces remains open. To this end, it is useful to compare our device to previous work on optical channel monitors that function based on defects from implant damage. The most salient basis for this comparison is the amount of photocurrent per unit waveguide length, Liu et al. (Y. Liu, C. W. Chow, W. Y. Cheung, H. K. Tsang, “In-Line Channel Power Monitor Based on Helium Ion Implantation in Silicon-on-Insulator Waveguides.” IEEE Photon. Technol. Lett. 18, 1882-1884 (2006)) have build a volume defect photodetector in silicon which achieves a quantum efficiency of 5% with around 2×10¹⁵ l/cm³ of He implants, but in a device that is 1.7 cm long. Our device achieves nearly the same efficiency in around 1/20 of the length. If we assume that there is one defect per Boron dopant in our device, and one defect per helium implant in Liu's device, then a substantial discrepancy exists. That is, with only half of the defects per unit volume, we achieve a similar quantum efficiency to Liu et al in a device that is times shorter. Other considerations only skew this comparison further. It is likely that there are, on average, multiple defects associated with a Helium ion implanted at 800 KeV as compared to including a dopant during the water manufacturing process. Moreover, our waveguide geometry has a substantial portion of the mode outside the waveguide, which would only serve to lower the influence of bulk defects.

On the other hand, both Liu et al. and Geis et al. both observed at least some photocurrent with undamaged silicon control samples, though much lower quantum efficiencies of approximately 0.1% were obtained. Geis et al. attributed this, without discussion or supporting data, to a process that occurred on the waveguide surface. We believe that they may have also observed surface state based absorption and generation of electron-hole pairs. The relative difference in efficiencies is probably due to the very different surface modal overlap that our waveguide exhibits.

The best basis for understanding the source of the photoconductive process is an analysis of the source of waveguide loss. It is believed that the generation of an electron-hole pair can be expected to correspond to the absorption of a photon and thus a certain amount of optical loss. Borselli et al. have recently shown that bulk silicon waveguide losses are no more than 0.13 dB/cm in p-type SOI. The SOI used was from SOITEC, and was doped with Boron in nearly identical concentrations to the wafers used for our devices. The fractional decrease in power due to hulk silicon loss over the course of a 1.5 mm device is only 0.45%, which would then be a strict upper bound on the quantum efficiency. Our devices exceed this limit by nearly an order of magnitude. This also strongly suggests that the defect states used by the photoconduction effect are likely to exist at the surface of the silicon waveguide, and are thought to be due to the etched silicon facets.

There are a number of experiments which could further confirm that surface states are the source of the photocurrent. In particular, one could build a detector device as described in this work, and then apply the surface treatment described by Borselli et al., which should remove many of the surface states. Another possibility is to do a more detailed study of the wavelength dependence of the photocurrent, in order to gain insight into what the underlying quantum process is. This was not possible with the current devices, however, due to the limited bandwidth of the grating couplers.

Calculation of the Surface State Density

The strength of the surface state absorption effect can be characterized by a surface state density, a in units of watts⁻¹s⁻¹ cm⁻¹. It identifies the product of the number of surface states and the optical absorption probability per unit of waveguide. The number of electron-hole pairs generated per second per cm of waveguide can be written as:

$\begin{matrix} {\frac{E\; H\; P}{s \cdot {cm}} = {\sigma\; I}} & {{Eqn}.\mspace{14mu}(10)} \end{matrix}$

Assuming a nearly uniform optical intensity throughout the detector, the responsivity for a device of length L can then be written: R=qσL  Eqn. (11)

Here q is the charge of an electron. Based on the responsivity of 36 mA/W for a device of length 1.5 mm, σ can be estimated at 1.5×10¹⁸ watts s⁻¹ cm⁻¹. This value is comparable to the surface state density calculated for similar samples with all optical measurements.

The waveguide loss can also be written as a function of the surface state density. α>=hvσ  Eqn. (12)

Here h is Planck's constant, and v is the optical frequency. The surface state density estimated above corresponds to a waveguide loss of 0.8 dB/cm. This is a significant fraction of the waveguide loss of 5 dB/cm that these waveguides typically exhibit.

We believe that with further optimizations of electrode geometries and processing parameters, improved responsivity can be obtained from surface-state absorption, at higher power and speeds. It is believed that the effect could be used to build a photodetector at 1 GHz or higher, which would be useful for commercial telecommunications applications. It is believed that surface-state absorption is an important component of waveguide losses in nano-scale silicon waveguides. It is believed that measurement of the photoconductivity of waveguide samples can provide a useful tool in loss optimization.

While the present invention has been particularly shown and described with reference to the structure and methods disclosed herein and as illustrated in the drawings, it is not confined to the details set forth and this invention is intended to cover any modifications and changes as may come within the scope and spirit of the following claims. 

1. An optical modulator configured to operate in a single-photon absorption mode, comprising: a substrate configured to absorb photons and to create charge carriers, said substrate configured to accept application of a voltage in a desired direction, said voltage when applied configured to move the charge carriers along said desired direction; a waveguide structure having a gate waveguide and at least one signal waveguide having an input port and an output port, said waveguide structure disposed adjacent said substrate in an orientation selected with respect to said direction of voltage, at least one of said substrate and said waveguide structure configured to operate in a single-photon absorption mode; said gate waveguide of said waveguide structure configured to receive a gate signal; said input port of said at least one signal waveguide configured to receive an input optical signal to be controlled in response to said gate signal; and said output port of said at least one signal waveguide configured to provide an output signal representative of said controlled input signal.
 2. The optical modulator of claim 1, further comprising a ring resonator.
 3. The optical modulator of claim 1, wherein said at least one signal waveguide is configured as one leg of a Mach-Zehnder interferometer.
 4. The optical modulator of claim 1, wherein said waveguide structure comprises a semiconductor material.
 5. The optical modulator of claim 4, wherein said semiconductor material is silicon.
 6. The optical modulator of claim 1, wherein said waveguide has surface state density, σ, of not less than 1.5×10¹⁸ cm⁻¹s⁻¹mW⁻¹.
 7. The optical modulator of claim 6, further comprising a ring resonator.
 8. The optical modulator of claim 6, wherein said waveguide structure comprises a semiconductor material.
 9. The optical modulator of claim 8, wherein said semiconductor material is silicon.
 10. The optical modulator of claim 1, wherein some portion of said waveguide structure having a surface to volume ratio of at least 18μm¹, computed using a unit length of 1 μm of said waveguide, with said width and depth dimensions of said waveguide being measured in units of microns.
 11. The optical modulator of claim 10, further comprising a ring resonator.
 12. The optical modulator of claim 10, wherein said at least one signal waveguide is configured as one leg of a Mach-Zehnder interferometer.
 13. The optical modulator of claim 10, wherein said waveguide structure comprises a semiconductor material.
 14. The optical modulator of claim 13, wherein said semiconductor material is silicon.
 15. The optical modulator of claim 1, wherein a material providing single-photon absorption is an alloy of one or more of silicon, germanium, and tin.
 16. The optical modulator of claim 1, wherein said material providing single-photon absorption comprises a dopant.
 17. The optical modulator of claim 1, wherein said optical modulator comprises a junction selected from a P-N junction and a P-I-N junction.
 18. A method of operating an optical modulator in a single-photon absorption mode, said optical modulator comprising a substrate configured to absorb photons and to create charge carriers, said substrate configured to accept application of a voltage in a desired direction, said voltage when applied configured to move the charge carriers along said desired direction; a waveguide structure having a gate waveguide and at least one signal waveguide having an input port and an output port, said waveguide structure disposed adjacent said substrate in an orientation selected with respect to said direction of voltage, at least one of said substrate and said waveguide structure configured to operate in a single-photon absorption mode; said gate waveguide of said waveguide structure configured to receive a gate signal; said input port of said at least one signal waveguide configured to receive an input optical signal to be controlled in response to said gate signal; and said output port of said at least one signal waveguide configured to provide as output an output signal representative of said controlled input signal; the method comprising the steps of: applying to said input port of said at least one signal waveguide an input optical signal; applying to said gate waveguide a gate signal representing a desired modulation to be applied to said input optical signal; and detecting at said output port of said at least one signal waveguide an output signal representing said input signal modulated using said gate signal; whereby said input signal is modulated by said optical modulator.
 19. The method of operating an optical modulator of claim 18, wherein said optical modulator further comprises a ring resonator.
 20. The method of operating an optical modulator of claim 18, wherein said at least one signal waveguide is one leg of a Mach-Zehnder interferometer. 